1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Introduction to energy problem - did I do this right?

  1. Dec 2, 2014 #1
    1. The problem statement, all variables and given/known data
    The bullet strikes a block of wood which exerts, on average, a force of 50,000N opposing the motion of the bullet. How far does the bullet penetrate?

    Mass of bullet = 25g
    Initial velocity of bullet = 350 m/s
    Final velocity of bullet = 0 m/s

    2. Relevant equations
    Ei+work=Ef
    K = ½mv^2
    Energy = Force * Displacement

    3. The attempt at a solution
    Please excuse my attempt of it's egregiously wrong, I was just introduced to energy yesterday; this is one of the first problems I'm doing.

    Anyways, because the final velocity of the bullet is zero, the bullet must have lost all of its energy, making Ef=0 J. The bullet initially had kinetic energy, making Ei=K. Thus, my new equation is
    K+work = 0 J
    ½mv^2+work=0 J

    I also know that the problem gave me how many Newtons oppose the object, but I need to find out how much energy it used. Since Energy = Force*Displacement, I can rewrite the equation as:
    ½mv^2+(F*Δx) = 0 J
    So I solved it below...
    ½(.025kg)(350 m/s)^2+(-50000N*Δx) = 0 J
    Δx=.03m

    Did I do this problem right? If not, what did I do wrong?
     
  2. jcsd
  3. Dec 2, 2014 #2
    I didn't check your arithmetic, but you methodology is correct.

    Chet
     
  4. Dec 2, 2014 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It's the right answer according to what the questioner seems to expect you to do, but the question is actually wrong.
    Knowing the average force you cannot deduce the distance, only the time. Your method effectively assumes a constant force. Average force is defined as ##\Delta##momentum/##\Delta##time. In general, this gives a different number from ##\Delta##KE/displacement.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Introduction to energy problem - did I do this right?
  1. Am i doing this right (Replies: 1)

  2. Am I doing this right? (Replies: 1)

Loading...